Radiographic imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low-energy X-ray from the generator, the details of the geometry and characteristics of the detector and the capacity of the acquisition system are all elements that affect how the actual reconstruction is performed.
Further, the measured X-ray intensity on a detector may include both scattering photons and primary photons. Thus, the images reconstructed from scattering (contaminated intensities) may contain scattering artifacts.
In one of many possible geometries, the X-ray source on top of the graph shown in FIG. 1 is emitting a X-ray beam forming a fan, traversing the object. While a wide range of values can exist, typically, the distance “C” is around 100 cm, “B” is around 60 cm, and “A” is around 40 cm. The principle of tomography requires that each point of the object is traversed by a collection of rays covering at least 180 degrees. Mathematical considerations show that the tomographic conditions are met when a scan of 180 degrees plus the fan angle is performed. In addition to the details of the scanner geometry and the detector behavior, the very nature of the X-ray interaction with the matter it traverses makes the problem more complex and requires another layer of correction and compensation.
For example, scattering is one of the major sources of discrepancy between the expected attenuation behavior and the measured data from a scanner without an anti-scatter grid or with a non-perfect anti-scatter grid. The naïve assumption that all the measured photons originate directly from the X-ray source is not exactly true. X-ray photons can be diverted from their original course in a purely elastic collision (Rayleigh scattering) or in a more complex inelastic collision (Compton scattering) in which both direction and energy are affected.
Several systems have been proposed to address scattering contamination. For example, most modern commercial scanners include an “anti-scatter” filter. This device is a collimation system exploiting the fact that all scattered photons will be diverted from their original path and will therefore enter the detector at a different angle from the photons coming directly from the X-ray tube, which is typically a small (e.g., less than one millimeter wide) point that is on the order of one meter away.
Energy discriminating detectors such as CdTe/CdZnTe-based photon-counting detectors may further experience changes in their responses to incident X-ray flux and energy spectrum due to effects such as temperature-drift, hysteresis associated with X-ray exposure and crystal polarization.